1. Field of the Invention
The present invention relates to a back-illuminated type solid-state imaging device, particularly, to a CMOS solid-state imaging device of a back-illuminated type and a method for manufacturing thereof.
2. Description of the Related Art
In the past, a CMOS solid-state imaging device of a back-illuminated type in which light is made incident from the rear surface side of a substrate to improve efficiency in using light and to obtain high sensitivity has been proposed as a CMOS solid-state imaging device. This back-illuminated type CMOS solid-state imaging device has a structure in which a CMOS transistor that constitutes a pixel is formed on the front surface side of a semiconductor substrate, a light receiving portion that becomes a photoelectric conversion element is formed to face the rear surface side of the substrate, and a multi-layer wiring is formed on the front surface side of the substrate, to make light incident from the rear surface side of the substrate. In this case, the rear surface on the opposite side to the front surface side where the multi-layer wiring of the semiconductor substrate is formed is polished to manufacture this solid-state imaging device.
In order to polish the semiconductor substrate stably, it is preferable that the thickness of a substrate is approximately 10 μm including a margin. Further, from a view point of sensitivity of red, it is preferable that the substrate has the thickness of this range. Here, in case of blue light and the like, since the photoelectric conversion is performed at a shallow position after the light is incident on the rear surface side of a silicon semiconductor substrate, it is necessary for a photoelectron to move a distance of approximately 10 μm until a charge accumulation portion on the front surface side.
On the other hand, a pixel pitch of, for example, 4 μm or less is not rare in recent years, and an aspect ratio (=substrate thickness/pixel pitch) becomes very large to be 3 or more in case of such minute pixel pitch. When the aspect ratio is large, it is difficult to manufacture a photoelectric conversion element of such shape and in addition, a photoelectron happens to enter an adjacent pixel while moving to the charge accumulation portion, which causes a crosstalk. Here, the crosstalk means that a signal of an adjacent pixel is mixed into a signal of an original pixel. When the crosstalk increases, a resolution becomes low and a color mixture occurs frequently in case of a CMOS solid-state imaging device of a single-plate type in which a color filter is attached to each pixel.
As a countermeasure for the above, Patent reference 1 proposes one in which an electric field is formed in a semiconductor substrate in a CMOS solid-state imaging device of a back-illuminated type. Since the electric field is formed, it becomes easy for a photoelectron to move in the depth direction of the substrate and the crosstalk can be reduced.
As another example of related art, the non-patent reference 1 has been presented.
[Patent Reference 1] Published Japanese Patent Application No. 2003-338615
[Non-patent Reference 1] Process and Pixels for High Performance Imager in SOI-CMOS Technology (authors: Xinyu Zheng, Sures hSeahadri, Michael Wood, Chris Wrigley, and Bedabrate Pain) presented in Session 3 of 2003 IEEE Workshop on Charge-Coupled Devices and Advanced Image Sensors